Smokers adjust their smoking behavior to maintain a certain level of nicotine. By affecting the level of nicotine that a smoker seeks to maintain, nicotine metabolism likely influences smoking behavior. The major pathway of nicotine metabolism is catalyzed by P450 2A6. Polymorphisms that decrease the amount or activity of 2A6 can alter nicotine metabolism. Polymorphisms reported to decrease 2A6 amounts or activity include CYP2A6 *2, *4, *7, *9, and *12. CYP2A6 *2 and *12 have allele frequencies of about 2% among Caucasians. Allele frequencies of CYP2A6 *4, *7, and *9 range from 3-24% among Japanese or Chinese. The goal is to identify major differences in nicotine metabolism attributable to variant CYP2A6 alleles, both in human volunteers and in liver samples. We will obtain a complete metabolic profile on 40 individuals (10-20 with at least one copy of a variant allele and 20 wild-type). Deuterium-labeled nicotine will be administered orally, and metabolites will be assessed by tandem mass spectrometry. In vitro, we will quantify the amount and activity of 2A6 and the relative contribution of 2A6 and 2B6 to nicotine metabolism in 10-20 livers with variant alleles and 20 wild-type livers. Minor pathways of nicotine metabolism will be characterized in vitro.